THE EFFECTS OF LOGGING AND FOREST REGENERATION ON WATER YIELDS IN A MOIST EUCALYPT FOREST IN NEW-SOUTH-WALES, AUSTRALIA

Water yields increased after logging by 150-250 mm per year in small catchments of moist old-growth eucalypt at Karuah in central New South Wales. The magnitude of this initial increase was directly related to the percentage of the catchment logged (29-79%). Where substantial vegetation removal took place in less than 20% of one catchment no increased water yield was observed. Water yields began to decline in all catchments 2-3 years after logging as regrowth eucalypts became established, and the rate of this decline was related to the mean stocking rate of eucalypt regeneration during the next 4 years. This water yield decline exceeded 250 mm in the sixth year after logging in the catchment with the highest stocking of regeneration and the highest regrowth basal area. Water yields in this catchment had declined to levels significantly below pre-logging levels by this time, supporting the notion that regrowth evapotranspiration had begun to exceed that of the old-growth forest. Patterns of declining water yield in the other catchments suggest that yields in some may also decline below pre-logging levels as regrowth evapotranspiration increases in line with increases in the basal area of the regrowth forest. Further study is required to determine the magnitude and duration of water yield reductions in these regrowth catchments, and to quantify the eucalypt growth rates and stand conditions responsible for the reductions. Nevertheless, these early results are consistent with water yield changes observed in mountain ash forest in Victoria, and support the concept of greater water use by a rapidly regenerating forest.